Pixels used in conventional image sensors are roughly classified into 3-transistor pixels, 4-transistor pixels, and 5-transistor pixels, according to the number of transistors included therein.
FIG. 1 to 3 show a typical pixel structure used in an image sensor, according to the number of transistors.
FIG. 1 shows a structure of a 3-transistor pixel. FIGS. 2 and 3 show a structure of a 4-transistor pixel.
Due to the existence of transistors in a pixel circuit, as shown in FIG. 1 to 3, a fill factor of a photodiode, that is an area for the photodiode with respect to a pixel size, is naturally reduced. In general, the fill factor ranges from 20 to 45%, considering capability of each semiconductor manufacturing process. Light that is incident onto the rest area of 55˜80% of the pixel is therefore lost. To minimize the loss of optical data, a subminiature lens is used for each unit pixel in a manufacturing process of an image sensor, so that the optical data can be condensed onto the photodiode of each pixel. A micro lens gain is defined as an increment of the sensitivity of a sensor using a micro lens with respect to the sensitivity of an image sensor without using the micro lens.
Given that the fill factor of a common diode is about 30-something percent, the micro lens gain is about 2.5˜2.8 times of gain. However, since a pixel size has decreased to 4 μm×4 μm, and even to 3 μm×3 μm. Further, with an emergence of a small-sized pixel of 2.8 μm×2.8 μm or 2.5 μm×2.5 μm, starting from when the pixel size is 3.4 μm×3.4 μm, the micro lens gain significantly drops from 2.8 times to 1.2 times of gain. This is caused by a diffraction phenomenon of the micro lens. The level of the diffraction phenomenon is determined by a function of a pixel size and position of the micro lens.
However, as the pixel size gradually decreases, the diffraction phenomenon of the micro lens becomes more severe, thereby dropping the micro lens gain less than 1.2 times of gain, which results in a phenomenon where the light condensation seems to be unavailable. This is newly being recognized as a cause of sensitivity deterioration.
In general, the decrease of the pixel size for the image sensor results in the decrease of the area for the photodiode. The area for the photodiode is normally closely related to the amount of available electric charge. For this reason, the amount of the available electric charge decreases when the size of the photodiode decreases. The amount of available electric charge of the photodiode is a basic feature of determining a dynamic range of the image sensor, and the decrease of the amount of available electric charge directly affects the image quality of the sensor. When the image sensor of which pixel size is less than 3.2 μm×3.2 μm is manufactured, its sensitivity decreases, and the dynamic range of the sensor with respect to light also decreases, thereby deteriorating the image quality.
An external lens is used in the process of manufacturing a camera module using the image sensor. In this case, light is substantially vertically incident onto a center portion of a pixel array. However, the light is less vertically incident onto edge portions of the pixel array. When an angle starts to deviate from the vertical angle by a predetermined degree, the light is condensed onto the micro lens which is out of the area, pre-assigned for condensation, for the photodiode. This causes a dark image, and more seriously, when the light is condensed onto a photodiode of an adjacent pixel, chromaticity may change.
Recently, with the development of the image sensor having from 0.3 million pixels and 1.3 million pixel to 2 million pixels and 3 million pixels, a dynamic zoom-in/zoom-out function as well as an automatic focus function are expected to be included in a mini camera module.
The feature of the functions lie in that the incident angle of the light significantly changes at edge portions while each function is performed. The chromaticity or brightness of the sensor has to be independent of changes in the incident angle. With the decrease of the pixel size, however, the sensor cannot cope with the changes in the incident angle. At present, the sensor can handle the automatic focus function, but the dynamic zoom-in/zoom-out function is not available yet. Therefore, it is difficult to develop a mini camera module providing a zoom function.